1. Field of the Invention
The present invention relates to deposition systems and more particularly to a method and system for vacuum deposition on large scale substrates in evacuated chambers.
Architectural glass which is transparent yet bears a reflective coating has been found highly desirable for use in buildings to reduce solar heating gains as well as for aesthetic purposes. Minor defects in the coatings are readily observable when the glass is installed and accordingly such glass must be provided with coatings which can be applied reliably without defects and remain securely adhered to the glass when it is in use.
Coating substrates, such as glass, by sputtering atoms of coating material onto them has been found to be an effective process for producing high quality relatively durable coatings. To assure optimum efficiency, the sputtering process should be conducted in a chamber under deep vacuum conditions (e.g., pressures of less than 50 microns of mercury). The atmosphere in the chambers should be substantially inert or otherwise chemically controlled, the chamber should be free from contamination and the substrates themselves must be virtually free from surface particles, contaminants, and static electricity to avoid irregularities and/or discontinuities in the coating.
2. Prior Art
The production of sputter coated substrates has been relatively widely used in the semiconductor industry where small scale production equipment can be used; however, because of the extremely deep vacuum pressures required for high quality sputter coating, production of relatively large scale coated substrates, like architectural glass, has required usage of large, expensive pressure vessels and production rates have been relatively limited.
An example of production equipment for coating small scale substrates is disclosed by U.S. Pat. No. 3,294,670 in which substrates are coated on a continuous production basis. These kinds of production facilities are constructed using minimum volume internal vacuum chambers so that appropriate pumps can evacuate them quickly and efficiently. Because of the relatively small size of the equipment the vacuum chamber wall areas are small and not subjected to great differential pressure forces. The continuous production technique tends to minimize the possibilities that substrates will be carrying surface dust, moisture, etc. when entering the sputtering chamber because the substrates can be individually cleaned just before the sputtering takes place.
When large scale substrates are to be sputter coated, problems arising from inefficient use of vacuum pumps, large chamber volumes and extreme differential pressure forces are encountered. Relatively large chamber volumes are necessitated by the substrate sizes and the chambers are thus not quickly evacuable to coating pressure levels of 50 microns of mercury or less. Different kinds of vacuum pumps must be operated in stages to evacuate the chambers to optimum coating pressure levels.
Mechanical vacuum pumps are effective to evacuate the chambers so long as the gas being pumped exhibits fluid flow characteristics. At pressure levels of from 700-500 microns the efficiency of the mechanical pumps is reduced dramatically because the movement of the remaining atmosphere in the chambers begins to take on molecular flow characteristics. This results in substantial reductions in pumping speed as the chambers continue to be evacuated to about 200 microns. Diffusion pumps can then be used to further evacuate the chambers to desired lower pressure levels.
Diffusion pumps, such as oil diffusion pumps, are ineffective when operated at pressures over 200 microns and therefore the chambers have had to be mechanically evacuated to the effective operating range of the diffusion pumps. The time taken to reduce the chamber pressure from 500 to 200 microns has been significant and reduces coating production rates appreciably.
Some production facilities for sputter coating relatively large glass lights have been proposed in which the glass is supported by racks in a large volume pressure vessel equipped with movable sputtering electrodes. The vessel is loaded, closed, and pumped down to the operating level after which the glass is coated, the vessel vented and reopened, and the coated glass removed. Examples of such facilities are disclosed by U.S. Pat. Nos. 3,907,660; 3,891,536; and 3,738,928.
These approaches attempt to reduce the adverse affects on production rates caused by the long pumping times required to suitably evacuate the vessels. In addition the vessels can be of cylindrical or semicylindrical shape which reduces the cost of their construction.
There are some practical drawbacks to these approaches. In addition to the length of time required to simply evacuate these vessels to their operating levels, the vessels are opened to atmosphere between coating operations and a large number of sheets of glass and their supporting structures are placed inside. This further extends the pumping time because substantial numbers of water and oxygen molecules, as well as other contaminants are introduced into and trapped by the vessel walls, the glass itself and its supporting structure. Such contaminants are gradually released and expelled as the pumping chamber pressure is reduced and maintained at a given level. The higher the chamber pressure remains during coating, the more likely it is that such molecules will be present in significant numbers during coating. Opening the vessel to atmosphere between coating operations and replacing the racks etc. replenishes the supply of these contaminants.
The interior of the chambers thus tends to be "dirty", even at exceedingly low pressures. The presence of these molecules can adversely effect the quality of the final coating. Purging the vessels of such molecules by maintaining the coating pressure level for a period of time before coating the glass is desirable; however, this further extends the cycle time.
Moreover, it is sometimes difficult to assure that the substrates remain clean before and during their assembly into the pressure vessels or on the supporting racks. That is to say, each substrate to be coated can not be cleaned immediately before being placed in the vessel and coated. The longer the substrates are exposed to ambient atmosphere and the more handling they receive the more likely it is that contamination will occur.
Attempts to increase the rate of production of coated glass have resulted in some more or less continuous coating facilities. One such proposal is disclosed by U.S. Pat. No. 3,925,182 in which a series of aligned chambers separated by pressure doors is provided and through which the glass is conveyed on suitable supports. The disclosed system employs a coating chamber with entrance and exit chambers on its opposite ends. The chambers are all about the same length and the equipment is designed so that the entrance and coating chambers communicate with each other as the coating process begins and the exit and coating chambers communicate as the coating process ends.
The chambers are mechanically evacuated with the entrance and exit chambers being pressure equalized, respectively, with the coating chamber at different times during each cycle to enable passage of the glass through the apparatus. The pumping time required for operating the equipment through a cycle tends to be reduced by providing minimum volume rectangular cross-section chambers and by operating the system at fairly high coating pressure levels (e.g. in excess of 100 microns); however, the exit and entrance chambers have to be pumped down from atmospheric pressure to the coating pressure level during each cycle. This is relatively time consuming because of the pumping inefficiency not withstanding the relatively small volume chambers.
The operation of this system requires back filling the entrance and exit chambers with inert gas in order to better assure a "clean" atmosphere in the coating chamber when it communicated with the entrance and exit chambers. Nevertheless contaminants can continue to be problems both of the relatively high coating pressure and because the entrance and exit chambers are opened to atmosphere and to the coating chamber during each cycle, enabling contaminants to enter the chambers with the entrance and exit of each batch of substrates and supports.
Providing additional chambers and/or lengthening the chambers relative to the maximum substrate length tends to increase the cost and complexity of the equipment to the extent that such installations are considered uneconomical. In particular, because the chamber volumes are kept as small as possible to increase the pumping rates, the differential pressure forces tending to crush the chamber walls are extremely great and require expensive pressure wall constructions.